1. Field of the Invention
The present invention generally relates to semiconductor laser driving units, optical scanners having the semiconductor laser driving units, and image forming apparatuses using the semiconductor laser driving units. In particular, the present invention relates to a semiconductor laser driving unit capable of supplying a driving current having an ensured overshoot amount or an ensured undershoot amount to a semiconductor laser, an optical scanner having the semiconductor laser driving unit, and an image forming apparatus such as a laser printer using the semiconductor laser driving unit.
2. Description of the Related Art
A known image forming apparatus such as a laser printer uses a semiconductor laser. In this case, the image forming apparatus generally has a semiconductor laser scanner and rotates a photosensitive drum to have a latent image formed thereon while scanning the drum along its shaft direction with a polygon scanner (e.g., polygon mirror).
FIG. 1 is a diagram showing an image forming apparatus described in JP-A-2001-096794 (Patent Document 1).
In FIG. 1, reference numeral 32 denotes a scanning and exposing apparatus (semiconductor laser scanner), reference numeral 22 denotes a photosensitive drum, and reference numeral 16 denotes a sheet onto which a toner image is transferred.
The semiconductor laser scanner 32 has a semiconductor laser (referred also to as laser diode (LD)) 50 and a rotating polygon mirror 52 that reflects a laser beam emitted from the semiconductor laser 50 as a light source and applies the laser beam to the photosensitive drum 22.
The semiconductor laser 50 is connected to a semiconductor laser driving unit 54 described below and controlled to emit the laser beam according to image data.
The laser beam emitted from the semiconductor laser 50 is converted from diffuse rays into parallel rays by a collimating lens 56 and incident on the rotating polygon mirror 52, which rotates in a direction as indicated by arrow D about a rotating shaft 60, via a cylinder lens 58.
The angle of the laser beam incident on each of reflecting surfaces 52A is continuously changed and deflected. Thus, the laser beam is scanned in the shaft-line direction (direction as indicated by arrow E, i.e., main scanning direction) of the photosensitive drum 22 to be applied thereto.
Using an f-θ lens 62 composed of a first lens 62A and a second lens 62B provided in the traveling direction of the laser beam reflected by the rotating polygon mirror 52, the semiconductor laser scanner 32 scans the photosensitive drum 22 with the laser beam at a constant speed and forms an image on the peripheral surface of the photosensitive drum 22. The laser beam that passes through the f-θ lens 62 is bent by a reflective mirror 64 to irradiate the photoreceptor body 22.
A mirror 66 is arranged on the upstream side of the traveling direction and the main scanning direction of the laser beam, and a photo detector 68 is arranged in a direction in which the laser beam is reflected by the mirror 66. The photo detector 68 detects an application start timing (i.e., SOS (Start of Scan)) for each line at which the semiconductor laser scanner 32 applies the laser beam to the photosensitive drum 22.
The semiconductor laser driving unit 54 has a bias current source from which a bias current having a prescribed current value is fed to the semiconductor laser 50, a switching current source from which a switching current having a prescribed current value is fed to the semiconductor laser 50, and an overshoot generation circuit that generates an overshoot at the rising of a semiconductor laser driving current.
Note that the value of the bias current is set to be less than the value of a threshold current required when the semiconductor laser 50 outputs coherent light. Further, the value of the switching current is set to be added to the value of the bias current to be greater than the threshold current required when the semiconductor laser 50 outputs the coherent light. In other words, the ON/OFF state of the semiconductor laser 50 is based on whether the switching current is fed.
The semiconductor laser driving unit 54 feeds a set driving current (Iop) to the semiconductor laser 50 and controls the semiconductor laser 50 to illuminate at a prescribed light amount.
FIG. 2 is a graph showing a relationship between the driving current of a semiconductor laser and the amount of light output from the semiconductor laser. As shown in FIG. 2, the semiconductor laser has the characteristic of outputting coherent light when a driving current Iop becomes equal to or greater than a threshold current (denoted as Ith) and changing its light amount with a constant inclination as the driving current Iop increases. Here, a light emitting current, which is fed after the threshold current Ith to obtain the light amount becomes 1 mW, is denoted as Iη.
Generally, the light output waveform of a semiconductor laser used in an image forming apparatus such as a laser printer is ideally a rectangular waveform as shown in FIG. 3. However, if a distortion occurs in the driving current Iop, a distortion also occurs in the light output waveform of the semiconductor laser. Consequently, the light output waveform of the semiconductor laser becomes a blunt waveform having a round shape at its rising as shown in FIG. 4.
In the case of the blunt waveform shown in FIG. 4, an average light amount of the semiconductor laser becomes smaller compared with the case of the rectangular waveform shown in FIG. 3 in proportion to the degree of the round shape, which results in the disorder of a dot image as a generated image and reduction in image quality. This phenomenon is remarkable if the output waveform of a semiconductor laser becomes blunt at its rising when the frequency of data and the duty of a pulse become small at the input of image data.
It is ideal to output the rectangular waveform shown in FIG. 3 but the driving current Iop cannot rapidly rise due to the presence of a parasitic capacitance. Therefore, the output of such an ideal rectangular waveform is not possible. In addition, since the semiconductor laser has a parasitic capacitance, the output waveform becomes the blunt light output waveform as shown in FIG. 4.
In order to avoid this problem, there has been often employed, as described in, for example, JP-A-2001-096794 (Patent Document 1) and JP-A-2006-068933 (Patent Document 2), a method of using a snubber circuit to constitute an overshoot circuit controlled by a pulse smaller than an image data pulse and overlapping an overshoot current with the driving current Iop to generate an overshoot at the rising of the driving current Iop.
Due to the generation of the overshoot, the driving current Iop becomes one as shown in FIG. 5. Accordingly, the driving current Iop can rise earlier. In addition, since the parasitic capacitance is charged earlier, the semiconductor laser can increase an average light amount in its light output waveform.
Further, it is generally ideal to output the rectangular waveform shown in FIG. 10 to drop the driving current Iop. In this case, however, an error in light amount may occur if the driving current Iop becomes blunt at its fall as shown in FIG. 6. Further, an error may occur in a next driving current Iop to reduce image quality if the driving current Iop is not secured until next data are given. Moreover, light may be erroneously emitted if the driving current Iop does not fall below the threshold current Ith.
It is ideal to output the rectangular waveform shown in FIG. 3 but the driving current Iop cannot rapidly fall due to the presence of a parasitic capacitance as in the case of rising the driving current Iop. Therefore, the output of such an ideal rectangular waveform is not possible.
In order to avoid this problem, there has been often employed, as a known technology, a method of attracting from the driving current Iop an undershoot current with which an undershoot is generated at the falling of the driving current Iop.
However, if the snubber circuit having a resistor and a capacitance is used outside a semiconductor chip, a manufacturing cost is increased with an increase in the number of components. Further, if the snubber circuit is incorporated in the semiconductor chip, the manufacturing cost is also increased with an increase in the area of the semiconductor chip.
In view of this problem, consideration is given to a circuit that generates an overshoot at the rising of a driving current and an undershoot at the falling of the driving current inside a semiconductor chip.
Here, FIG. 7 shows output waveforms corresponding to the respective sizes (Iop1, Iop2, and Iop3) of a prescribed driving current Iop when a constant overshoot current and a constant undershoot current are fed.
As shown in FIG. 7, a larger overshoot is generated if the prescribed driving current Iop is small (Iop1) while a smaller overshoot is generated if the prescribed driving current Iop becomes larger (Iop2 and Iop3).
Next, feeding of the overshoot current and the undershoot current is described in detail.
FIG. 8A shows the basic configuration of a driver unit, and FIG. 8B shows a relationship between a changed portion ΔVgs in the gate voltage of the driver unit and the driving current.
As shown in FIG. 8A, assuming that the parasitic capacitance of the driving unit is C, a rising time is t, the overshoot current overlapped with the driving current is ΔI, and the changed portion of a gate voltage is ΔVgs, a relationship between them can be expressed by the following formula.ΔI×t=C×ΔVgs 
It is clear from this formula that the changed portion ΔVgs of the gate voltage depends on the overshoot current ΔI because the parasitic capacitance C and the rising time t are constant. That is, as shown in FIG. 8B, since the changed portion ΔVgs of the gate voltage set to the driving current Iop is changed with the constant feeding of the overshoot current, the output waveform of the overshoot is changed. This also applies to the output waveform of the undershoot.
Further, the threshold current Ith, the light emitting current Iη, and the driving current Iop necessary for setting a light amount are changed depending on the type of a used semiconductor laser. Furthermore, even if the same type of a semiconductor laser is used, it has a large individual variation and is varied due to temperature or the like. Therefore, the driving current Iop is not constant even if the semiconductor laser is caused to emit a constant amount of light. For this reason, the overshoot current and the undershoot current must be changed according to the size of the driving current Iop.
Here, as shown in FIG. 8B, since the value of the gate voltage Vgs depends on transistor characteristics due to the settings of the driving current Iop, the driving current Iop and the gate voltage Vgs are not merely in a proportional relationship. Accordingly, the overshoot current and the undershoot current are not simply proportional to the driving current Iop.
Therefore, the present invention may have an object of providing a semiconductor driving unit capable of supplying a driving current having an ensured overshoot amount or an ensured undershoot amount to a semiconductor laser regardless of the size of the driving current.
Moreover, as shown in FIG. 9, the driving current Iop may be configured to include a bias current (referred to as Ibi) and a switching current (referred to as Isw).
FIG. 10 shows a relationship between the driving current Iop and the gate voltage Vgs according to a change in the bias current Ibi when the switching current Isw is constant.
If the same switching current Isw is supplied when the bias current Ibi is small (Ibi1) and the bias current Ibi is large (Ibi2), a comparison between the gate voltages ΔVgs1 and ΔVgs2 producing the driving currents Iop1 and Iop2, respectively, shows that the gate voltage ΔVgs2 is smaller than the gate voltage ΔVgs1. Therefore, the overshoot current must be decreased as the bias current Ibi increases when the switching current Isw is constant.
Also, in the case of the falling of the switching current Isw, the undershoot current must be decreased as the bias current Ibi increases.
Therefore, the present invention may have an object of providing the semiconductor laser driving unit capable of supplying a driving current having an ensured overshoot amount and/or an ensured undershoot amount to a semiconductor laser regardless of the size of a bias current.
Next, FIG. 11 shows a relationship between the driving current Iop and the gate voltage Vgs according to a change in the switching current Isw when the bias current Ibi is constant.
If the small switching current (Isw1) and the large switching current Isw (Isw2) are supplied with the bias current Ibi, a comparison between the gate voltages ΔVgs1 and ΔVgs2 producing the driving currents Iop1 and Iop2, respectively, shows that the gate voltage ΔVgs2 is greater than the gate voltage ΔVgs1. Therefore, the overshoot current must be increased as the switching current Isw increases when the bias current Ibi is constant.
Also, in the case of the falling of the switching current Isw, the undershoot current must be increased as the switching current Isw increases.
Therefore, the present invention may have an object of providing the semiconductor laser driving unit capable of supplying a driving current having an ensured overshoot amount or an ensured undershoot amount to a semiconductor laser regardless of the size of a switching current when a bias current is constant or regardless of the size of the bias current when the switching current is constant.
Further, the present invention may have an object of providing the semiconductor laser driving unit that supplies a driving current having an ensured overshoot amount or an ensured undershoot amount to a semiconductor laser regardless of the sizes of a bias current and a switching current.
Patent Document 1: JP-A-2001-096794
Patent Document 2: JP-A-2006-068933